The book is packed with wonderful, pedestrian insights about urban life: Why are some neighbourhoods completely dormant, while others are oozing with life? How do you revive a street, and how can you tell if it's moribund (to use one of Jane's favourite words)? What causes a street to feel safe, and what doesn't? It is easiest to think of the book as a collection of observations made by Mrs. Jacobs while sitting with a cup of tea on a NYC sidewalk.
For a street to be safe, it needs life. To gain life, it needs a continues flow of people throughout the day. For that,, it needs a diversity of shops, open and alive at different times of day: Perhaps a café, bakery, popular lunch spot, postal office, pharmacy, an apparel shop, a few restaurants, and a pool bar. In order for the street to have a diversity of merchants, it needs varied prices of rent -- only chains can afford completely new builds. If everything's new, only chains will be able to afford the rent to move in (think of a strip mall, or completely new neighbourhood -- ever seen an antique store there?).
The book's filled with such observations, but they're buried in lengthy, tiring prose. The sidewalk chapters are worth reading in full, but after that I mostly read the first and last few pages of each chapter to decide whether to read the full thing. The observations have stood the test of time, but the writing has not.